The following text should not be construed as an admission of knowledge in the prior art. Furthermore, citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention, or that any reference forms a part of the common knowledge in the art.
Proton conducting fuel cells (PCFCs), and other intermediate temperature protonic ceramic electrochemical devices (IT-PCECDs) exhibit several advantages over traditional solid oxide fuel cells (SOFCs) in terms of lower operation temperature (300° C.-600° C.) and higher efficiency. Among proton conducting ceramics, the recently reported proton conductor BaCe0.7Zr0.1Y0.1Yb0.1O3-δ (BCZYYb) has shown particularly promising performance in single-cell fuel cell demonstrations at test temperatures around 750° C. However, the maximum power density of the PCFCs achieved (˜1100 mW/cm2 and at 750° C.) was much lower than that of SOFCs.
Very few promising performances for PCFCs at temperatures lower than 600° C. have been reported. One of the challenges for the development of high performance, intermediate temperature PCFCs lies in the discovery of appropriate cathode materials. The poor performance of most PCFCs is attributed, in part, to the use of cathodes that were developed for SOFCs operating at much higher temperatures between about 700° C. and 1000° C. whereas the target PCFC operation temperatures are near 500° C. One reason for this is that the application of conventional SOFC cathodes, which are based on either electron-conducting oxides or mixed oxygen ion and electron-conducting oxides, to electrolytes developed for PCFCs restricts the cathode reaction only to points where the electrolyte and cathode phases meet.
Although mixed oxygen ion and electron conducting oxides and proton conducting oxides have been researched extensively, none have been promising. For example, although yttrium-doped barium zirconates (BZY) are excellent proton conductors and also exhibit some oxygen-ion conductivity in dry reducing atmospheres, its electronic conductivity is extremely small. However, it is unquestionable that a prerequisite for a promising intermediate temperature PCFC cathode is high electronic conductivity. Similarly, while BaCo0.4Fe0.4Zr0.2O3-δ (BCFZ) provides a strong electrochemical performance and a good stability, making it compatible with BCZYYb electrolytes, its low proton transport limits the cells performance.
Accordingly, there exists a need in the art for an intermediate temperature PCFC cathode material having a high electronic conductivity, high oxygen ion transport, high proton transport, and good compatibility with PCFC electrolytes.
There is also a need for a stable cathode for low temperature SOFCs. Cathodes that operate at low temperatures can have complications. For example, SOFCs must demonstrate excellent long-term durability and thermal robustness in addition to good performance in commercial applications. Poor thermal cycling stability is usually caused by poor thermal shock resistance due to mismatches in thermal expansion characteristics between the various components of the membrane electrode assembly (MEA) and/or stress-induced delamination between electrode and electrolyte. In most SOFCs, the cathode is usually sintered separately and at lower temperature compared with anode and electrolyte in order to get a porous structure with high surface area. However, this separate sintering can lead to a weak electrode/electrolyte interface that is susceptible to delamination. Despite the crucial importance of thermal-cycle stability, few studies in the literature have examined rapid thermal cycling in SOFCs. As a notable exception, Kun Joong Kim (Kim et al., Micro Solid Oxide Fuel Cell Fabricated on Porous Stainless Steel: A New Strategy for Enhanced Thermal Cycling Ability, Sci. Rep., 6, 22443 (2016), which is incorporated by reference) recently demonstrated good stability after 10 quick thermal cycles for micro SOFCs fabricated on porous stainless steel.
These and other issues are addressed with the present invention.